Microsoft Knowledge Base Article 898715 contains the following introduction:

Microsoft Windows Installer version 3.1 was included with Microsoft Windows Server 2003 SP1. This article discusses how to obtain an update for Windows Installer for the 32-bit and 64-bit editions of Windows Server 2003 SP1 and for the 64-bit editions of Windows XP. You can use this update on both IA-64-based systems and x64-based systems.